1. Field of the Invention
This invention relates to a guard assembly for the nose of a chain saw.
2. Description of the Prior Art
A well known safety problem in the use of a chain saw is the occurrence of "kickback". This is a violent movement of the saw which can occur when the nose, that is, the forward end of the saw, contacts a solid object. The conditions giving rise to kickback are as follows. The chain of the saw tends to bite into an object with a force proportional to the speed of the chain. At the same time, there is a corresponding reaction force exerted on the saw by the object. If the momentum of the chain and other moving parts of the saw carries the chain into the object, then the energy of the chain is expended in the cutting action and the reaction force on the saw is greatly reduced. However, at lower chain speeds, the momentum may not be sufficient to sustain the cutting action. In fact, the chain may become seized by the object. If the chain is not carried on through a cut, the saw is suddenly subjected to the full reactive force. When this force is exerted at the middle or on the upper quadrant of the nose of the saw, the nose moves suddenly upward in rotation generally about the rear of the saw.
Because of the way that a chain saw is held, the kickback motion is extremely difficult for the operator to control; thus there is considerable risk that he will be hit by the moving chain. If the saw is being held horizontally at the time of the kickback, then its motion will be somewhat opposed by the weight of the saw. This opposition decreases substantially as the saw is held more nearly vertically. For this reason, perhaps the most dangerous kickback situation is when the saw is being used to cut an object overhead. There may be additional lack of control in this situation if the operator's arms are extended to reach the object above.
One approach to minimizing the injurious effects of kickback is to employ a safety brake on the saw which quickly stops it when kickback occurs. A number of such brakes have been developed. This is a relatively elaborate and expensive partial solution to the problem. It stops the chain, but it does not prevent rotation of the saw. There is also the possibility that the saw will be damaged. Even under the best of circumstances, kickback represents a significant inconvenience.